Use of a photographically useful agent in such a condition that it is previously incorporated in a photographic material and so contrived that its effect will be produced at the time of necessity contains various features different from those presented by using it in a state of the addition to a processing solution. Specific examples of the features in the former case are as follows: The incorporation in a photographic material enables effective utilization of photographic agents of the kind which tend to decompose under the acid-alkaline or the oxidation-reduction condition and consequently, can not withstand the longtime storage in a processing bath and at the same time, makes it possible to simplify a composition of the processing solution to be employed associated therewith and thereby to facilitate the preparation of the processing solution. Further, this makes it possible to force a required photographic agent to function in desired time during the photographic processing or at only the desired place, that is, in only a specified layer and the neighboring layers of a multilayer photographic material. Furthermore, this permits the presence of a photographic agent in the photographic material in such an amount as to vary as a function of silver halide development. However, if a photographic agent is added to a photographic material in its active form, it becomes impossible to make the photographic agent exhibit its ability to the expected degree because during storage before the photographic processing, it reacts with other components contained in the photographic material or it is decomposed by heat, oxygen or so on. One method for solving the above-described problem involves adding a photographic agent to a photographic material in such a form that its active group is blocked and turned photographically inactive, that is, in a form of its precursor. Such a method can have various advantages in various cases to which it is applicable. For instance, in the case where the useful photographic agent is a dye, blocking a functional group of the dye which has a great effect on its spectral absorption characteristic results in a shift of is spectral absorption band to shorter wavelengths or to longer wavelengths and therefore, even if the dye is present in a silver halide emulsion layer having the spectral sensitivity in the wavelength region corresponding to the absorption band which the dye has in the unblocked state, a lowering of the sensitivity due to the so-called filter effect can be prevented. In another case where the useful photographic agent is an antifoggant or a development restrainer, blocking of their active groups makes it possible to suppress desensitization arising from adsorption of these agents to light-sensitive silver halide grains and formation of silver salts upon storage and at the same time, release of these agents at required times permits the reduction of fog density without being attended by a drop in the sensitivity, the prevention of fog due to overdevelopment, development stoppage at a desired time, and so on. In still another case where the useful photographic agent is a developer, an assistant developer or a fogging agent, if their active or adsorptive groups are blocked, various, photographically adverse effects which arise from semiquinonnes and oxidants produced by air oxidation upon storage can be prevented, and generation of fogging nuclei upon storage can also be prevented because injection of electrons into silver halide grains can be inhibited. Therefore, stable processings can be effected therein. In a further case that the useful photographic agent is a bleach accelerating agent or a bleach-fix accelerating agent, it becomes also possible to prevent reactions with other components copresent in the photographic material from occurring upon storage by blocking its active group and that, to make its expected ability bring into full play at a desired time by removing the blocking group.
As described above, a precursor of photographic agents can be utilized as an extremely valuable tool in bringing out abilities of the photographic agents to the best advantage. However, their precursor must satisfy very severe requirements for the purpose of practical use. That is, it must be one which can satisfy two requirements contradictory to each other; one consists in ensuring stable presence of the precursor under a storage condition, and the other consists in setting its blocking group loose at a desired time upon the processing and in releasing the photographic agent rapidly and that, efficiently.
A number of techniques for blocking a photographic agent have already been known. For instance, a technique using a blocking group such as an acyl group, a sulfonyl group or the like is described in published examined Japanese Patent Application No. 44806/72 (corresponding to U.S. Pat. No. 3,615,617): one which utilizes such a blocking groups as to release a photographic agent by the so-called reversal Michel's reaction is described in published examined Japanese Patent Application Nos. 39727/79 (corresponding to U.S. Pat. No. 3,674,478), 9696/80 (corresponding to U.S. Pat. No. 3,791,830) and 34927/80 (corresponding to U.S. Pat. No. 4,009,027): one which utilizes such a blocking group as to release a photographic agent with the production of quinone methide or its analogs by intramolecular electron transfer is described in published examined Japanese Patent Application Nos. 39727/79, 135944/82, 135945/82 and 136640/82: one which utilizes an intramolecular ring-closure reaction is described in published unexamined Japanese Patent Application No. 53330/80 (corresponding to U.S. Pat. Nos. 4,358,525 and 4,310,617): one which utilizes cleavage of a 5- or 6-membered ring is described in published unexamined Japanese Patent Application Nos. 76541/82 (corresponding to U.S. Pat. No. 4,335,200), 135949/82 (corresponding to U.S. Pat. No. 4,350,752) and 179842/82: and so on. However, these known techniques suffer from some defect that, for example, although stable under a storage condition, some precursor requires a high alkaline condition such as pH higher than 12 for the processing because the photographic agent-releasing rate thereof is too slow; some precursor decomposes gradually to cause a failure of its function as the precursor upon storage under moderate conditions even though it can release the photographic agent at a sufficiently fast rate by the processing under mild conditions such as in the pH range 9-11; some precursor allows little latitude in controlling the rate of releasing the photographic agent therefrom and therefore, it requires a very narrow pH range for effecting the processing; or so on.